1. Field of the Invention
The present invention relates to a method for removing hydrogen chloride from a flue gas.
2. Description of the Related Art
A device for treating of a flue gas containing hydrogen chloride is installed, for example, in a waste treatment equipment of subjecting waste to a incineration treatment. In this waste treatment equipment, a first dust collector and a second dust collector are arranged in series, and after removal of dust such as combustion fly ash contained in flue gas by the first dust collector, a dechlorination of flue gas is carried out in the second dust collector.
To carry out dechlorinating in the second dust collector, a dechlorinating agent is added into flue gas before the second dust collector. As a dechlorinating agent, a calcium-based dechlorinating agent such as calcium hydroxide (Ca(OH)2) has conventionally been mainly employed. Calcium hydroxide, if added into flue gas, reacts with hydrogen chloride (HCl) contained in the flue gas to generate residue of dechlorination containing calcium chloride (CaCl2), calcium oxide (CaO) and the like. However, thus generated residue of dechlorination is useful only in that calcium chloride is applied as a snow melting agent or a moisture absorbent, with a narrow range of effective uses. Residue of dechlorination is mostly solidified through a chemical treatment or with cement and disposed of in reclamation. However, acquisition of reclamation site is now becoming more difficult.
It is therefore proposed to use a sodium-based dechlorinating agent such as sodium hydrogencarbonate (sodium bicarbonate: NaHCO3) or sodium carbonate (soda ash: Na2CO3) in place of the calcium-based dechlorinating agent. In this case, when a sodium-based dechlorinating agent is added into a flue gas, hydrogen chloride contained in the flue gas becomes sodium chloride (NaCl). Adding water to residue of dechlorination dissolves sodium chloride. Therefore, water-soluble constituents dissolved in water are diluted and discharged, and only water-insoluble constituents not dissolved in water are separated and can be subjected to a combustion treatment in a melting furnace, thus eliminating the necessity of disposal in reclamation.
When sodium hydrogencarbonate is adopted as a sodium-based dechlorinating agent and sodium hydrogencarbonate has a particle size larger than 30 μm, powder particles never coagulating together between them, and the agent is stable as powder. However, sodium hydrogencarbonate having a particle size larger than 30 μm leads to a very low removing ratio of hydrogen chloride, so that the use thereof as a dechlorinating agent is not appropriate. In general, therefore, sodium hydrogencarbonate ground to a particle size of 30 μm or less is used as a sodium-based dechlorinating agent.
However, when sodium hydrogencarbonate is ground to a particle size of 30 μm or less, coagulation of powder particles results in a form of fibrous dust balls or stone-like lumps. Ground sodium hydrogencarbonate has thus an unstable condition as powder, thus making is impossible to stably supply the same to flue gas.
To solve this defect, it is the usual practice to use an anti-caking agent. In the conventional art, a hydrophobic anti-caking agent has been used as such an anti-caking agent. A hydrophobic anti-caking agent brings about a remarkable solidification inhibiting effect. Sodium hydrogencarbonate added with a hydrophobic anti-caking agent has high flowability and floodability property, and exhibits satisfactory stability as a powder.
However, when a hydrophobic anti-caking agent added with sodium hydrogencarbonate is added into a flue gas as a dechlorinating agent, particles resulting from reaction with hydrogen chloride after calcination of sodium hydrogencarbonate, viz residue of neutralization, and particles of the anti-caking agent, which both have a high flowability, tend to easily entrap inside a filter cloth attached in a second dust collector comprising, for example, a bag filter. When particles of residue of neutralization or particles of the anti-caking agent penetrate into the filter cloth, there is caused clogging, resulting in an excessive pressure drop at the filter cloth and making it impossible to continue operation. It is difficult to recover from clogging even by back washing of the bag filter by the use of pulse air.
Further, some of particles produced from reaction with hydrogen chloride after calcination of sodium hydrogencarbonate, viz residue of neutralization, having entrapped into the filter cloth and particles of the anti-caking agent pass through the filter cloth, causing leakage of the dechlorinating agent and residue of neutralization. A double-woven glass cloth is usually used as a filter cloth. In order to prevent leakage of the dechlorinating agent and residue of neutralization, it is necessary to use a special filter cloth made by applying a Teflon membrane coated to the surface of the double-woven glass cloth. If the membrane is damaged or peeled off during use, however, a new problem of leakage of chemicals from this portion is encountered.
An object of the present invention is to prevent occurrence of an excessive pressure drop or leakage in the filter cloth attached to the dust collector.